Various microorganisms are known to have a beneficial effect on plants, e.g. bacteria of the general Rhizobium. Bacillus, Azotobacter, Arthrobacter, Pseudomonas, Azospirillum and Cyanobacteria (blue-green algae), and mycorrhizal fungae. Such microorganisms are introduced to the plants by the use of inoculant compositions. Although the following description is mainly concerned with Rhizobium inoculant compositions, it will be appreciated that similar principles apply to the use of other microorganisms.